Starting from a microscopic model of interactions in A + C-60 crystals, where A+ is an alkali metal ion and C-60 the molecular ion, we present a theory of the phase transitions from the orientationally disordered Fm3m structure to polymer and dimer phases. The electronic charge transfer is accounted for by the t1u molecular orbitals of C-60. The resulting Coulomb interactions between neighboring C-60 and between C-60 and the sublattice of alkali metal ions lead to new orientation dependent potentials which have to be added to the intermolecular pair potential and to the crystal field. By studying the orientation dependence of the crystal field and the molecular field, we find that, in comparison with the phase transition Fm3m→Pa3 known from C60-fullerite, additional channels to a Pmnn structure with subsequent polymerization and dimerization are opened. We study the symmetry of the low temperature phases and compare with experimental results. The present work comprises an extension of the formalism of rotator functions to molecular orbitals.
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